New Sass architecture and translation system

Taking care of #13.

Everything seems to work according to what we talked about in the relevant issue. However, please note that I did not translated anything into English yet. Maybe @pierrechoffe might be interested in contributing with this.

Also, I moved every tiny bit of configuration regarding each theme (`advice`, `error`...) in a Sass map. It's much easier this way to retrieve configuration for each message type, as well as making the code both elegant and ordered.

I think it's a good start. Please feel free to ask me to update anything in this PR that would be okay yet.